In short-distance optical interconnects applications, low energy consumption as well as high transmission speed of the building block devices is becoming the key technological issues as the data transmission bandwidth increases. Thus, the figure of merit is energy consumption per transmitted bit. According to a recent technology roadmap provided in D. A. B. Miller, “Device requirements for optical interconnects to silicon chips”, Proceedings of the IEEE, vol. 97, p. 1166 (2009), a few 10 s fJ/bit is required in 2015-2020 for light transmitters of chip-level optical interconnects.
As light emitter, vertical-cavity surface-emitting lasers (VCSELs) are one of the preferable existing solutions. This is because their fabrication technology is matured and their energy consumption is much smaller than that of edge-emitting lasers due to their small active material volume. To send a bit signal, output light intensity of a light emitter should be modulated. There are two schemes for modulating the output light intensity; direct modulation and indirect (or external) modulation.
In the direct modulation scheme, the current injection to a laser is modulated. This leads to the intensity modulation of the output light. A state-of-the-art result is reported in Y.-C. Chang and L. A. Colden, “Efficient, high-data-rate, tapered oxide-aperture vertical-cavity surface-emitting lasers”, IEEE Journal of selected topics in quantum electronics, vol. 15, p. 704, (2009). The transmission speed was 35 Gb/s, the energy consumption excluding the RF driver circuitry was 12.5 mW, and the emission wavelength was 980 nm. The demonstrated energy per bit of 357 fJ/bit (=12.5 mW/35 Gb/s) is remarkably small but is not sufficient for the aforementioned applications. The weakness of this approach is that it is difficult to further increase the speed or reduce the energy consumption: Speed of a laser diode is decided by its intrinsic response and circuit response. The intrinsic speed is defined by −3 dB bandwidth of the intrinsic frequency response which is proportional to relaxation oscillation frequency, fr:
                              f          r                ∝                                            I              -                              I                th                                                    V              p                                                          (        1        )            where I is the injection current, Ith, threshold current, and Vp, modal volume. In order to obtain higher intrinsic speeds, the injection current needs to be higher while the modal volume, preferably smaller. In the demonstrated VCSEL, the modal volume is not likely to be further reduced since the oxide aperture diameter of 3 μm is very small. Regarding the injection current, if one increases the current for a higher intrinsic speed, it will result in higher energy consumption. On the other hand, if one decreases the current for smaller energy consumption, it will result in slower intrinsic speed. Thus, it is difficult to further increase the speed and decrease the energy consumption simultaneously in the conventional VCSEL structure. One should also consider that high injection current is detrimental to long-time stability of small-volume lasers. The speed related to the circuit response is mainly decided by the series resistance and capacitance of the laser structure. In the demonstrated VCSELs, these parasitic terms were already tightly suppressed. Thus, a significant improvement in speed related to parasitic circuit terms is not expected.
In the externally modulated scheme, constant-intensity light is generated in the laser part and the intensity modulation of this light occurs in an integrated modulator part. Since no modulation occurs in the laser part, current injection to the laser part can be small, resulting in small energy consumption of the laser part. Thus, if the energy consumption of the modulator part is small as well, the energy consumption of the whole structure including both the laser and modulator parts can be low.
A number of references disclose such an approach. For example, in U.S. Pat. No. 7,593,436 part of the light-exiting distributed Bragg reflector (DBR) includes an electrooptic material. Thus, the reflectivity spectrum of this light-exiting DBR can be modulated by modulating the reverse-biased voltage across the electrooptic material. This modulation of reflectivity spectrum leads to allowing and shutting the light emission, i.e., intensity modulation of the light output. A state-of-the-art result obtained by the inventors of the aforementioned invention is reported in V. A. Shchukin, et al., “Ultrahigh-speed electrooptically-modulated VCSELs: Modelling and experimental results,” Proceeding of SPIE, vol. 6889, 68890H, (2008). The energy per bit for the laser part was 40-80 fJ/bit at injection currents of 1-2 mA while that for the modulator part was about 100 fJ/bit. Here, the modulation speed was 40 Gbit/s and the lasing wavelength was about 960 nm. Thus, the overall energy per bit was 140-180 fJ/bit which is fairly lower than that of the direct modulation approach, but still needs further reduction to meet the required specification. In addition, another limitation that needs to be noted is that this approach of using a DBR which embeds an electrooptic material is not feasible for long wavelength VCSELs (wavelength 1310 nm): In order to obtain a sufficient contrast in reflectivity at a lasing wavelength of interest, the stopband widths of the passive DBR without an electrooptic material and the active DBR with an electrooptical material should be almost same. For long wavelength VCSELs, dielectric DBR or GaAs/AlGaAs DBR with large stopband widths can be used for passive DBRs. But, for active DBRs showing electrooptic effect, one needs to use InP-based material which has a much smaller stopband width than that of dielectric or GaAs/AlGaAs DBRs.
Hence, an improved way of modulating the laser output would be advantageous, and in particular a more efficient way of providing modulated laser outputs at very fast modulation rates and low energy consumption would be advantageous. In addition, a way that works both at short and long wavelengths is advantageous.